1. Field of the Invention
In many processes which have one or more steps requiring the distribution of fibrous or particulate material it has proven convenient and economical to use a liquid vehicle, particularly water, for this distribution. Subsequent removal of the liquid vehicle (drying) in such cases becomes an important consideration. While drying usually can be accomplished simply by exposure to the atmosphere and evaporation, speed of operation and other economic factors in most cases dictate the use of accelerated drying techniques. Examples of these techniques are well-known and include contact with a hot surface such as a steam heated dryer drum, contact with a gas stream which may be heated such as air, and contact with a solvent in which the liquid to be removed is highly soluble, causing it to be drawn from the material being dried and concentrated into the solvent.
Since solvent-drying is usually more expensive due to additional chemical costs and often capital costs for solvent recovery equipment, its use is primarily limited to processes which require its unique advantages. Contact with a hot drum in most cases results in an ironing out or flattening of the dried material, and air drying of fibrous materials frequently imparts a harsh stiffness to the material. Solvent-drying, on the other hand, can result in a soft, relatively uncompressed, dried product.
2. Description of the Prior Art
In conventional solvent-drying steps, the material to be dried is usually drawn in web or fiber form through a solvent bath. The liquid to be removed is exchanged in the bath leaving the web or filament drier as it emerges from the solvent bath. In some cases it has proven desirable to employ multiple solvent baths to increase the rate of liquid removal or where different solvents are required to remove more than one liquid, for example. In copending and co-assigned U.S. patent application Ser. No. 402,311 to Frederick O. Lassen entitled "Filaments of Chemically Modified Cellulose Fibers, Method of Making Such Filaments and Webs and Products Formed Therefrom" there is disclosed a solvent-drying process wherein multiple solvent baths are utilized with the result that solvent renewal requirements can be substantially reduced.
It is a characteristic of solvent-drying steps that periodic changing of the solvent bath is required. The evaporation of the solvent combined with the build-up of the liquid being renewed will eventually render the drying properties of the bath ineffective. In most cases it is necessary to separate the liquid removed from the solvent in order that the solvent can be reused. This presents a problem of substantial economic importance in virtually all processes utilizing a solvent-drying step. The entire bath volume must be treated by distillation or other separation techniques which inherently causes a considerable loss of solvent. It can be readily seen, that a significant amount of the expense of solvent-drying is occasioned by the solvent regeneration or reconcentration step. The present invention is directed at a method of solvent-drying which greatly facilitates solvent renewal and reduces the solvent losses.